Process for oxidation of xylenes



Patented Oct. 8, 1.957

PROCESS FOR OXIDATICN F XYLENES Emil Keunecke and Karl-Heinz Imhausen, Witten (Ruhr), Germany, assignors to Imhausen Werke G. on. b. IL, Witteu (Ruhr), Germany, a corporation oi Germany No Drawing. Application May 22, 1953, Serial No. 356,900

Claims priority, application Germany May 24, 1952 5 Claims. (Cl. 260-524) This invention relates to the oxidation of xylenes to yield aromatic carboxylic acids. More particularly, it relates to the oxidation of xylenes to yield toluic acids.

It is known that xylenes can be oxidized to carboxylic acids by elemental oxygen or oxygen-containing gases in the presence of catalysts, such as heavy metal salts, in the liquid phase. Under these conditions, the corresponding toluic acids are obtained in addition to relatively small amounts of the corresponding dicarboxylic acids. The oxidation proceeds at a considerable rate at a suitable temperature, e. g., 120 C. If technical xylenes which have been purified in the usual way, e. g., by treatment with sulfuric acid, alkali washes, and redistillation are used, an induction period of from a few minutes up to about half an hour is observed at the beginning, after which the oxidation proceeds for a longer time at a rather uniform rate. The length of the induction period is also dependent on the oxidation conditions; at low temperatures, for example, it is longer than in the case of high temperatures. In general, however, this short induction period signifies no noteworthy impairment of the oxidation. However, it is frequently observed that the oxidation of the xylenes, upon attainment of a cer tain degree of oxidation, slows to a marked degree or stops entirely. Further introduction of oxygen, the addition of fresh catalyst, or a change in the oxidation conditions, results in no improvement. The slowing or stopping of the oxidation often occurs even when only relatively small amounts of xylenes are oxidized to acids, e. g., after conversion of 25% or less of the xylenes into monocarboxylic acids. The reasons for this behavior of the xylenes have not been explained.

Upon closer investigation of the course of the oxidation, the surprising fact was discovered that the decrease in the rate of oxidation in the course of the oxidation of xylenes can be avoided by suitable pretreatment of the xylenes to the extent that it is possible to maintain the rate of oxidation at a high value even after the formation of considerable quantities of acid. For example, if technically pure xylenes are treated with relatively small amounts of anhydrous AlCls, expediently at a somewhat elevated temperature, and the reaction products thus formed are mechanically separated from the xylenes, it is observed that in the case of a xylene pretreated in this way, although there also is a short induction period, the oxidation proceeds more rapidly and at a greater rate for a considerably longer time so that oxidates with a considerably increased content of oxidation products are obtained.

Therefore, the process according to the invention consists in that the xylenes to be subjected to oxidation with oxygen or oxygen-containing gases, preferably in the pres ence of catalysts, such as heavy metal salts, in particular cobalt salts, under pressure if occasion arises, in the liquid phase, are pretreated with relatively small amounts (about 0.5 to 3%) of AlCls at a normal or slightly elevated temperature (up to about 80 C.) and subsequently separated mechanically from the AlCls sludge.

Suitable catalysts for the oxidation step are those known for oxidation with gaseous oxygen, especially the salts of heavy metals which, like cobalt or manganese, can occur in several v'alences. Particularly suitable have been found the salts of cobalt with toluic acid, as well as with lower fatty acids, especially the mixture of first run fatty acids having from 6-12 carbon atoms obtained in the oxidation of parafiin.

The temperature at which the oxidation of the xylene With oxygen or oxygen-containing gases is carried out may vary within wide limits; it is practically between about C. and 250 C., advantageously between about C. and 200 C. The rate of reaction is increased by raising the pressure; a pressure increase also makes it possible to raise the reaction temperature without any need of giving up operation in liquid phase.

Example The behavior of technically pure xylene upon oxidation was investigated by oxidizing 1,000 parts with 20 parts of cobalt salts of forerun fatty acids from paraifin oxidation (C6 to C10 fatty acids) with 100 liters of air per kilogram of xylene per hour at C. at normal pressure. The air was introduced by means of a perforated tube into the reaction vessel which was provided with a high speed stirrer.

A sample of this pure xylene was submitted to the same oxidation conditions after it had been stirred for 15 minutes at about 60 C. with 2% anhydrous AlCla, decanted from most of the sludge, and filtered from the remainder after addition of about 2.5% of dry fullers earth. The oxidation proceeded under the same conditions in the fashion indicated by the table.

Acid N um- Acid Num- Time, Hours ber of the her of the Untreated Pretreated Xylene Xylene The xylene carried away by the stream of air in the oxidation was considered in the determination of the acid number.

The technically pure xylene used in this example contained about 28% p-xylene, about 55% tit-xylene and about 15-p-o-xylene, the remainder being ethylbenzene and paraflinhydrocarbons. Technically pure xylene isomers may be used in place of the mixtures if desired.

What we claim and desire to protect by Letters Patent is:

l. A process for the oxidation of xylenes to organic acids which comprises the steps of slurrying xylene with anhydrous AlCls in the amount of from about 0.5 to 3.0% by weight of the xylene at a temperature not exceeding 80 C. until a sludge is formed, mechanically separating the sludge from the xylene, and oxidizing the thus treated xylene in liquid phase with an oxygen-containing gas at an elevated temperature in the presence of an oxidation catalyst.

2. The process of claim 1 wherein p-xylene is thus treated.

3. The process of claim 1 wherein m-xylene is thus treated.

4. The process of claim 1 wherein o-xylene is thus treated.

5. A process for the oxidation of xylenes to organic acids which comprises the steps of slurrying xylene with anhydrous AlCla in the amount of 2% by weight of the 3 xylene at a temperature of about 60 C. for 15 minutes whereby a sludge is formed, mechanically separating said sludge from the xylene, and oxidizing the thus treated xylene in liquid phase with an oxygen-containing gas at an elevated temperature in the presence of an oxidation catalyst.

References Cited in the file of this patent UNITED STATES PATENTS 1,284,887 Gibbs Nov. 12, 1918 OTHER REFERENCES Thomas: Aluminum Chloride in Organic Chemistry, pp. 25, 652, and 717-718 (1941).

Eglofi et al.: Isomerization of Pure Hydrocarbons, pp. 356-358 (1942). 

1. A PROCESS FOR THE OXIDATION OF XYLENES TO ORGANIC ACIDS WHICH COMPRISES THE STEPS OF SLURRYING XYLENE WITH ANHYROUS ALCL3 IN THE AMOUNT OF FROM ABOUT 0.5 TO 3.0% BY WEIGHT OF THE XYLENE AT A TEMPERAURE NOT EXCEEDING 80* C. UNTIL A ''SLUDGE IS FORMED, MECHANICALLY SEPARATING THE SLUDGE FROM THE XYLENE, AND OXIDIZING THE THUS TREATED XYLENE IN LIQUID PHASE WITH AN OXYYGEN-CONTAINING GAS AT AN ELEVATED TEMPERATURE IN THE PRESENCE OF AN OXIDATION CATALYST. 